1. Field of the Invention
The present invention relates to methods of removing impurities from carbonaceous materials, and more particularly to methods of removing sodium, potassium and other alkali and alkaline earth metals from PAN materials.
2. History of the Prior Art
It is known in the art to provide products made from carbonaceous materials in various forms such as fibers which have been woven into a fabric and carbonized. For certain applications such as ablative components on aerospace vehicles and the like, such products in addition to being carbonized should have high temperature thermal stability, low thermal conductivity and low concentration of alkali and alkaline earth impurities (low ionization potential materials) to reduce the electron concentrations in the boundary layer and thereby the attenuation of vehicle communication which results therefrom. For such applications alkali and alkaline earth metals must be considered impurities, and as such are desirably removed from the carbonaceous material as completely as possible. The removal of alkali and alkaline earth metal impurities also significantly increases the oxidation resistance of such materials so as to enhance their usefulness in high temperature material applications.
There are various ways of producing carbonaceous materials having the desirable low content of alkali and alkaline earth metal impurities. One way is to heat the material within the graphitization temperature range. This has proven to be undesirable in that while it volatilizes and thereby eliminates most or all of the alkali impurities, it also leaves the material with a crystalline structure of a graphitic nature such that the thermal conductivity thereof is unacceptably high. Another approach is to make the carbonaceous product from precursor material which itself has a very low alkali and alkaline earth metal content. However, such precursor materials are very expensive to produce and are not found among standard commercially available materials. A third approach, which is the one most often used for economical reasons, is to treat the carbonaceous material at some stage in the processing thereof prior to carbonization to remove a substantial portion of the alkali and alkaline earth metal impurities. This enables the material to be subsequently carbonized at low enough temperatures so as not to adversely affect the crystalline structure and thereby the thermal conductivity of the final product while at the same time providing a relatively pure product of substantial carbon composition.
U.S. Pat. No. 3,413,094 of Gibson, issued Nov. 26, 1968 and assigned to the assignee of this application, provides one example of a method of treating fibrous carbonaceous materials so as to eliminate alkali and alkaline earth metal impurities. The methods of the Gibson patent involve dipping material which has been carbonized in an aqueous solution of hydrobromic acid or hydroiodic acid and thereafter firing the treated products at a temperature sufficient to remove substantial metallic impurities but below a temperature sufficient to substantially increase thermal conductivity and crystallinity of the product. Following the acid dip, the material is full of alkali and alkaline earth metal impurities, and the extra firing step is required in order to eliminate such impurities. However, the extra firing step increases the chance of crystallization of the material and resulting higher thermal conductivity in the finished product.
The methods of the Gibson patent were developed in conjunction with the widespread use of cellulosic precursor materials such as rayon, and such methods are unsatisfactory when used with materials of polyacrylonitrile origin. This appears to be due in part to the distribution and chemical bonding of alkali and alkaline earth metal ions throughout the thickness of the fibers in the case of polyacrylonitrile material. Such impurities in materials of cellulosic origin are not an intrinsic chemical part of the fibers and are therefore relatively easily removed by various washing and scrubbing techniques.
The availability of polyacrylonitrile precursor materials in recent years and the resulting popularity thereof in terms of low cost and other factors have created a need for a method of purification capable of reducing the total alkali and alkaline earth metal content of carbonized material to levels on the order of 30 parts per million or less.
It is generally known that certain types of carbonaceous materials can be purified to some extent by washing in acids, detergents, or even pure water. An example of a treatment involving washing with both detergent and acid is provided by U.S. Pat. No. 3,179,605 of Ohsol, issued Apr. 20, 1965. The Ohsol patent is concerned with purification of regenerated cellulosic fibers for purposes of enhancing their general properties such as tensile strength rather than alkali and alkaline earth metal impurity. This is done by washing the fibers in a non-ionic detergent, then rinsing in water, then washing in an aqueous acid solution, then rinsing in water, then drying, and finally carbonizing the fibers.
The Ohsol patent is typical of prior art methods of treatment which are complex and which do not produce acceptable levels of final purity for ablative applications even when used to purify materials of cellulosic origin. When such methods are used to process carbonaceous material of other than cellulosic origin such as materials made from polyacrylonitrile precursors, the resultant laundering of some surface impurities and little else has little effect on the total impurity content. While the patent discusses purity levels on the order of 10-25 parts per million of sodium in conjunction with its cellulosic precursor material, the total alkali and alkaline earth metals content is much larger and becomes even higher as the material is carbonized.
U.S. Pat. No. 2,950,253 of Kling et al. provides a further example of a washing or laundering process for removing surface impurities. Kling et al disclose a variety of different chemicals for cleaning soiled fabric.
U.S. Pat. No. 4,079,446 of Horikiri et al. which is of interest with respect to acid treatment of materials addresses the problem of the instability of polyethylene at high temperatures. Polyethylene is very difficult to form into a fiber and readily loses fiber integrity upon heating. In order to overcome this obstacle in forming a carbon fiber out of polyethylene fiber, the polyethylene is treated with an acid to preserve fiber integrity. The acid is such that it sulfonates the fiber. Accordingly, Horikiri et al do not deal with the removal of alkali and alkaline earth metals from fibers such as polyacrylonitrile but instead address the specific problem of the poor integrity of polyethylene fibers at high temperatures and the fact that such integrity can be improved by sulfonating the fiber through interaction with an acid.
A further patent which is of interest with respect to acid treatment of materials is U.S. Pat. No. 4,113,847 of Fukishima et al. The Fukushima et al patent relates to a process for making acrylonitrile precursor material in which a spun mixture of fibers including acrylonitrile is washed and then stretched in hot acid water having a pH below a specified level. Thus this patent relates to the production of PAN precursor having good filament separability, no breakage of single filaments and few fluffs and little disorder of filaments, and not to the purification of PAN material which has already been produced.
U.S. Pat. No. 2,932,550 of Walmsley is of interest for its disclosure of the treatment of PAN material with an acid. However, such treatment has nothing to do with purification and instead is performed to create dye sites. Thereafter, a dye chemically combines with the PAN material, which is the desired result. The PAN material is immersed in sodium carbonate, reinforcing the fact that Walmsley is not concerned with the presence of sodium or other alkali or alkaline earth metals.
U.S. Pat. No. 3,412,062 of Johnson et al. and U.S. Pat. No. 3,532,466 of Johnson et al. are of interest in describing conventional processes for carbonizing polyacrylonitrile material.
Other developments of interest in this area include that shown by U.S. Pat. No. 4,073,869 of Kalnin in which material of at least 90% carbon composition is treated with a strong acid to add oxygen to the chemical change and thereby reduce thermal and electrical conductivities, the acid having a concentration of at least about 65% and preferably on the order of 100%. Japanese Pat. No. 49-109633 describes the use of nitric acid to set a polymer during spinning. Japanese Pat. No. 48-42812 describes treatment of carbonized or graphitized fibers with acid to increase porosity and surface area and enhance strength. Japanese Pat. No. 49-26195 describes the use of acid to set fibers during spinning so as to produce a fiber for cation exchange processes. Four different articles by Takahashi in "Chemical Abstracts", Vol. 64, 1966, 8367b-c, 12, 862b-c describe various treatments of flame-proofed PAN material including heat treatment, moisture sorption, stability upon exposure to chemicals including weak and strong acids and the effects of acid treatment on tensile strength.
An example of a method of purifying PAN-based material which advances over the methods previously discussed is provided by a co-pending application of Gary D. Shepherd et al., Ser. No. 140,257, filed Apr. 14, 1980, now U.S. Pat. No. 4,388,289, and commonly assigned with the present application. The Shepherd et al application describes a method of removing alkali and alkaline earth metal impurities from PAN material which has been oxidized but not carbonized to any extent. The purification method involves contacting the oxidized PAN material with an aqueous acid solution at an elevated temperature followed by rinsing with a solvent that is substantially free of alkali and alkaline earth metal ions.
At the time that the process described in the Shepherd et al application was developed, it was known that PAN material which had already been carbonized was virtually impossible to purify from a practical standpoint. Carbonization appears to seal in the alkali and alkaline earth metal impurities to such an extent that the material must be heated to a temperature on the order of 1600.degree. C. or greater to achieve the desired levels of purity. High temperatures on that order result in unacceptably high thermal conductivity for ablative applications of the material. It was therefor assumed that purification had to take place before any carbonization of the material was begun.
In the purification method described in the Shepherd et al application the PAN material which has been oxidized is first woven into a fabric before being contacted with the aqueous acid solution and rinsed with a solvent. Formation of a fabric of the PAN material is necessary if the material is to be purified in an economical fashion. The fabric is continuously advanced through the various cells of a tank containing the aqueous acid solution. Following exposure to the aqueous acid solution, the fabric is slowly advanced through a rinse tank containing the solvent and is then dried. Following drying, the purified fabric is carbonized by being advanced through a carbonization furnace heated to an appropriate carbonization temperature for a period of several minutes or longer to provide a product of substantially carbon composition.
Following oxidation the PAN material which is typically in the form of a plurality of tows is woven into a fabric as noted above. This not only facilitates purification of the PAN material but it also provides a fabric of essentially carbon composition when carbonization has been completed. Unfortunately carbonization of the oxidized and purified tows results in a weight loss in the tows of up to 50% and a shrinkage in the tows of about 25-30%. Consequently, at the end of carbonization the fabric is of a generally porous, sleazy nature and lacks the structural integrity desired for use or further processing of the fabric.
Accordingly, it would be desirable to provide an improved method of producing purified carbonacious material from polyacrylonitrile precursor material. In particular it would be advantageous to provide a method of producing purified carbonaceous material in which weight loss and shrinkage of the individual tows comprising a fabric as a result of carbonization of the fabric is reduced so as to provide a fabric of substantially carbon composition which is of relatively low porosity and otherwise has relatively good structural integrity.